Dehydrodechlorination of Methylene Chloride, Chloroform, and Chlorodiphenylmethane in the Presence of Ga/N Lewis Pairs

Transmetalation occurs
upon addition of GaCl3 to (quinolin-8-yl)­trimethyl­stannane.
The compound dissolves immediately in pyridine, and recrystallization
gives dichloro­pyridinyl­(quinolin-8-yl)­gallium­(III).
In chloroform, the compound bis-μ-(quinolin-8-yl)-μ-chloro-dichloro­digallium­(III)
tetra­chloro­gallate could be isolated in small quantities;
however, the major product was trichloro­(quinolinium-8-yl)­gallate­(III)
zwitterion. The zwitterion also formed upon addition of methylene
chloride or chloro­diphenyl­methane. We hypothesize that
the highly electrophilic digallyl cation abstracts chloride to form
a carbocation and that proton transfer from the carbocation to the
quinoline nitrogen affords transient carbenes. In particular, diphenyl
carbene forms from dehydro­dechlorination of chloro­diphenyl­methane
in toluene/​cyclohexene to give a well-defined mixture of products
due to cyclopropanation and C–H insertion reactions. Dichloro­pyridinyl­(quinolin-8-yl)­gallium­(III)
undergoes reaction with chloroform only at elevated temperature to
yield quinolinium tetra­chloro­gallate salt as the product.
This salt also forms in the reaction of chloroform with GaCl3 and quinoline at elevated temperature. The zwitterion could be converted
to quinolinium tetra­chloro­gallate upon heating, which
supports the idea that it was formed initially as an intermediate.
Thus, the Ga/N Lewis pairs appear capable of dehydro­dechlorination
of chloroalkanes.